The experimental evidence concerning large-scale magnetic fields
suggests that magnetic fields should have similar strength
over different length-scales. In this
sense inflationary mechanisms seem to provide
a rather natural explanation for the largeness of the correlation scale.
At the same time, the typical amplitude of the obtained seeds is, in
various models, rather minute.

Primordial magnetic fields can also
be generated through physical mechanisms operating
inside the Hubble radius at a given physical time.
Particularly interesting moments in the life of the
Universe are the epoch of the electroweak phase transition
(EWPT) or the QCD phase transition where
magnetic fields may be generated according to different
physical ideas. In the following
the different proposals emerged so far will be reviewed.

At the time of the EWPT the typical size of the Hubble radius is of the
order of 3 cm and the temperature is roughly 100 GeV. Before
getting into the details of the possible electroweak
origin of large-scale magnetic fields it is useful to present a
kinematical argument based on the evolution of the correlation scale
of the magnetic fields
[142].

Suppose that, thanks to some mechanism, sufficiently
large magnetic fields compatible with the critical density
bound are generated inside the Hubble radius at the electroweak epoch.
Assuming that the typical coherence length of the generated magnetic
fields is maximal, the present correlation scale will certainly be much
larger but, unfortunately, it does
not seem to be as large as the Mpc scale at the epoch of the
gravitational collapse. As already mentioned, the
growth of the correlation scale may be enhanced, by various processes
such as inverse cascade and helical inverse cascade. For instance, if
the injection spectrum generated at the electroweak epoch is
Gaussian and random a simple estimate shows that the present correlation
scale is of the order of 100 AU which is already larger than what
one would get only from the trivial expansion of length-scales (i.e. 1 AU)
[142].
If, in a complementary perspective, the injection spectrum is strongly
helical, then the typical correlation scale can even be of the order of
100 pc but still too small than the typical scale of the gravitational
collapse.

Large-scale magnetic fields can be generated at the electroweak epoch in
various ways. Consider first the case when the phase transition
is strongly first order.

Hogan [152]
originally suggested the idea that
magnetic fields can be generated during first-order phase
transitions. Since during the phase transition there are gradients in the
radiation temperature, similar thermoelectric
source terms of MHD equations (which were discussed in the context
of the Biermann mechanism) may arise. The magnetic fields, initially
concentrated on the surface of the bubbles, are expelled
when bubbles collide thanks to the finite
value of the conductivity.

The idea that charge separation can be generated during first-order
phase transitions has been exploited in
[242].
The suggestion
is again that there are baryon number gradients at the phase boundaries
leading to thermoelectric terms. In the process of bubble
nucleation and collisions turbulence is then produced. In spite
of the fact that the produced fields are sizable, the correlation scale, as
previously pointed out, is constrained to be smaller than 100 pc.

In a first-order phase transition the phases of the complex order
parameter of the nucleated bubbles are not correlated. When the bubbles
undergo collisions a phase gradient arises leading to a source terms for
the evolution equation of the gauge fields. Kibble and Vilenkin
[243]
proposed a gauge-invariant difference between the phases of the
Higgs field in the two bubbles. This idea has been investigated in the
context of the Abelian-Higgs model
[244,
245,
246].
The collision of two spherical bubbles in the Abelian-Higgs model leads
to a magnetic field which is localized in the region at the intersection
of the two bubbles. The estimate of the strength of the field
depends crucially upon the velocity of the bubble wall.
The extension of this idea to the case of the standard model
SU(2) × U(1) has been discussed in
[247].
A relevant aspect to be mentioned is that the photon field in the broken
phase of the electroweak theory should be properly defined. In
[248]
it has been shown that the definition employed in
[247]
is equivalent to the one previously discussed in
[249].

It has been argued by Vachaspati
[250]
that magnetic fields can be
generated at the electroweak time even if the phase transition
is of second order. The observat on is that, provided the Higgs field
fluctuates, electromagnetic fields may be produced since the gradients of
the Higgs field appear in the definition of the photon field
in terms of the hypercharge and SU(2) fields.
Two of the arguments proposed in
[250]
have been scrutinized in subsequent discussions.
The first argument is related to the averaging which should
be performed in order to get to the magnetic field relevant
for the MHD seeds. Enqvist and Olesen
[251]
noticed that if line averaging is relevant the obtained magnetic field
is rather strong. However
[251]
(see also [150])
volume averaging is the one relevant for MHD seeds.

The second point is related to the fact that the discussion of
[250]
was performed in terms of gauge-dependent quantities. The problem is
then to give a gauge-invariant definition of the photon field in terms
of the standard model fields. As already mentioned this problem has been
addressed in
[247]
and the proposed gauge-invariant is equivalent
[248]
to the one proposed in
[249].

In [252]
a mechanism for the generation of magnetic fields at the
electroweak epoch has been proposed in connection with
the AMHD equations. The idea is to study the conversion of the
right-electron chemical potential into hypercharge fields. In this
context the baryon asymmetry is produced at some epoch prior to
the electroweak phase. The obtained magnetic fields are rather strong (i.e.
|| ~
1022 G at the EW epoch) but over a small scale , i.e.
10-6Hew-1 dangerously close to
the diffusivity scale.

The final point to be mentioned is that, probably, the electroweak
phase transition is neither first order nor second order but it is
of even higher order at least in the context of the minimal standard
model [253,
254].
This conclusion has been reached using non-perturbative techniques and
the relevant point, in the present context, is that for Higgs boson
masses larger than mW the phase transition seems
to disappear and it is possible to pass from the symmetric to the broken
phase without hitting any first or second order phase transition.

There have been also ideas concerniing the a possible generation
of magnetic fields at the time of the QCD phase transition occurring
roughly at T ~ 140 MeV, i.e. at the moment when free
quarks combine to form colorless hadrons. The mechanism here
is always related to the idea of Biermann with thermoelectric
currents developed at the QCD time.Since the strange quark is heavier
than the
up and down quarks there may be the possibility that the quarks develop a
net positive charge which is compensated by the electric charge in the
leptonic sector.
Again, invoking the dynamics of a first-order phase transition, it is
argued that the shocks affect leptons and quarks in a different way so
that electric currents
are developed as the bubble wall moves in the quark-gluon plasma.
In [255]
the magnetic field has been estimated to be
|| ~ G
at the time of the QCD phase transition and with
typical scale of the order of the meter at the same epoch.

In [256,
257]
it has been pointed out that, probably,
the magnetic fields generated at the time of QCD phase transition may be
much stronger than the ones estimated in
[255].
The authors of
[256,
257]
argue that strong magnetic fields
may be generated when the broken and symmetric phase of the theory
coexist. The magnetic fields generated at the boundaries between quark
and hadron phases can be, according to the authors, as large as
106 G over scales of the
order of the meter at the time of the QCD phase transition.

Recently, in a series of papers, Boyanovsky, de Vega and Simionato
[258,
259,
260]
studied the generation of large scale magnetic fields during a phase
transition
taking place in the radiation dominated epoch. The setting is a theory
of N charged scalar fields coupled to an Abelian gauge field that
undergoes a phase transition at a critical temperature much larger than the
electroweak scale. Using non-equilibrium field theory techniques the
authors argue that
during the scaling regime (when the back-reaction effects are dominant)
large scale magnetogenesis is possible. The claim is that the minimal
dynamo requirement of Eq. (6.1) is achievable at the electroweak
scale. Furthermore, much larger magnetic fields can be expected if the
scaling regime can be extended below the QCD scale.